196 



P TERA SPID OMORPHI 



the entire surface of the scales and plates of the Pteraspidae 

 (Fig. 166, E, I). 



Thus we seem to be able to trace a genetic connection between 

 the very aberrant Pteraspids and some early kind of shark, through 

 the Coelolepidae ; but whether the four families given below really 

 form a phyletic series remains doubtful. 



The Pteraspidomorphi are among the most ancient of vertebrate 



dc. 



D 



JL b " ,r. A 



FIG. 166. 



Diagrams of the structure and development of the dermal skeleton of A, an early stage, and 

 B, later stages of Elasmobranchs ; C, Thelodus ; D, Psammoslevs ; E, Pteraspis, all in section 

 at right angles tc the surface ; the dentine is black. F-I, enlarged views of the outer 

 surface of the dermal skeleton of, F, Thelodus head ; G, Thelodus tail ; H, Psammosteus shield ; 

 and I, Ptera.tpis shield, bf, expanded basal plate ; b.m, basement membrane ; c.t, connective 

 tissue ; d.c, dentine cap ; cp, epidermis ; 7, bony lamellae ; p, pulp-cavity ; /, surface ridge ; tr, 

 bony trabeculae of vascular layer. 



animals ; they flourished in Upper Silurian and Devonian times, 

 beyond which they do not appear to have survived. 



Family COELOLEPIDAE. The body is flattened dorso-ventrally, and 

 expanded on each side into a longitudinal flap of considerable size. A 

 single dorsal fin is present. The exoskeleton is in the form of numerous 

 scattered placoid scales ; either sharp, conical, and tooth-like as in 

 LanarJcia (Fig. 167), or more flattened and closely fitting as in Thelodus 

 (Fig. 168). No lateral-line system has been made out (Traquair [466], 

 Rohon [371]). 



Thelodus, Ag. ; Coelolepis, P. ; Upper Silurian and Devonian, Europe. 

 Lanarkia, Traq. ; Upper Silurian, Scotland. 



